1. Field of the Invention
The present invention relates to a light source of a non-selfluminous display device, and more particularly to a liquid crystal display device having a backlight which includes a light guide plate and uses an LED as a light source.
2. Background Art
Recently, the liquid crystal display device has been popularly used as a display device. Particularly, the liquid crystal display device is used as a display part of portable equipment because the liquid crystal display device is thin and light-weighted, and consumes small electric power.
However, the liquid crystal display device is not self-luminous and hence, the liquid crystal display device requires a lighting means. In general, as a lighting device which is used for the liquid crystal display device, a planar lighting device referred to as a backlight is popularly used. Conventionally, although a cold cathode fluorescent lamp has been used as a light emitting element (also referred to as a light source) of the backlight, an LED (light emitting diode) has been also recently used as the light emitting element.
The backlight includes a plate-shaped light guide plate. A material of the light guide plate is a light transmitting resin or the like, and light incident on the light guide plate from the light emitting element propagates in the inside of the light guide plate. A reflection/scattering member such as grooves, projections or a printed material is formed on the light guide plate, and the light which propagates in the inside of the light guide plate due to such a reflection/scattering member is directed and radiated toward a liquid crystal display device side.
When LEDs are used as a light emitting element, there arises a drawback that a thickness of the LEDs is larger than a thickness of the light guide plate. Accordingly, as disclosed in JP-A-11-260136, for example, there has been proposed the constitution of a light guide plate which changes a thickness thereof in two stages, wherein a thickness of the light guide plate is made large at a light entering surface on which light is incident from a light source, and the thickness of the light guide plate at a light radiation surface is set smaller than the thickness of the light guide plate at the light entering surface thus forming an inclined surface extending toward the light radiation surface from the light entering surface. However, the technique described in JP-A-11-260136 is provided for preventing leaking of light by making use of an angle of the inclined surface, and the technique does not take leaking of light from the inclined surface when a thickness of the light radiation surface is further decreased or a method of forming the light guide plate into consideration.